We are studying cell lines isolated from both normal breast and breast carcinomas to determine whether mammary epithelial cells (MEs) can be infected by HIV-1. MEs are a potential reservoir for HIV and may be a source of the HIV detected in the breast milk of seropositive mothers. HIV-1 (MN) was used to infect six different ME cell lines (as well as H9 cells for comparison), which were monitored for virus production by p24 antigen assay, DNA PCR analysis and immunofluorescence. These infected cultures have been maintained for over 60 days and are continuing in culture. Low level infection is detectable by p24 assay and by DNA PCR. The time course of infection as measured by p24 antigen varied among different cell lines. Experiments are in progress to 1) ascertain whether these cells are producing infectious virus and to 2) identify the cells and/or cellular factors which may be involved in maintaining a latent HIV infection.